1. Technical Field
The present invention relates to a hydraulic shock absorber with a damping force adjustment unit.
2. Related Art
For instance, there are hydraulic shock absorbers provided in a motorcycle to suspend a rear wheel of the motorcycle in relation to a vehicle body which have a configuration in which a cylinder is mounted on one of a vehicle body side and an axle side, a piston rod is mounted on the other side, the piston rod is partially inserted in the cylinder, a piston fixed to an end portion of the piston rod that faces an inside of the cylinder is brought into sliding contact with an inner circumference of the cylinder in a slidable manner, and a suspension spring is interposed between the cylinder and the piston.
In these hydraulic shock absorbers, some have a damping force generation unit for generating damping force by utilizing flow of oil in the cylinder that is caused when the piston rod moves in and out of the cylinder, and a damping force adjustment unit for adjusting the damping force (see, for example, Patent Literature 1 (JP-A-2008-014431)). A comparative hydraulic shock absorber with the damping force adjustment unit is described hereinafter with reference to FIG. 7.
FIG. 7 is a vertical cross-sectional view of the comparative hydraulic shock absorber provided with the damping force adjustment unit. In the hydraulic shock absorber 101 in the view, part of a piston rod 103 mounted on an axle side is inserted from below into a cylinder 102 mounted on a vehicle body side, wherein a piston 112 is fixed to a tip end of the piston rod 103 that faces an inside of the cylinder 102, and is brought into sliding contact with an inner circumference of the cylinder 102 in a vertically slidable manner, and a suspension spring, not shown, is interposed between the cylinder 102 and the piston rod 103. Note that the inside of the cylinder 102 is divided by the piston 112 into an upper chamber S1 and a lower chamber S2 which are filled with oil, the working fluid.
The piston 112 is provided with an extension side damping valve 119 and a compression side damping valve 120 which configure the damping force generation unit, and the damping force adjustment unit is provided inside the piston rod 102. The damping force adjustment unit has the following configuration.
In other words, the damping force adjustment unit is configured by a push rod 133 inserted in the hollow piston rod 103 so as to be able to move up and down, an electric motor (stepping motor) 134 which is a driving source, and a feed screw mechanism 135 for converting rotation of an output shaft 134a of the electric motor 134 into linear movement (up-and-down movement) of the push rod 133, and an oil path 136 is formed in an upper end portion of the piston rod 103, between the piston rod 103 and the push rod 133. A ring-shaped seating member 141 is attached to an upper end opening of the piston rod 103, and a side hole 137 in the piston rod 103, immediately below the piston 112. The lower chamber S2 and the oil path 136 are communicated with each other by this side hole 137.
Thus, for example, during an extension stroke in which the piston rod 103 moves downward along with the piston 112 with respect to the cylinder 102, the oil in the lower chamber S2 of the cylinder 102 is compressed by the piston 112, increasing its pressure. Consequently, the oil in the lower chamber S2 pushes the extension side damping valve 119 of the piston 112 open to flow into the upper chamber S1, resulting in generation of damping force in the hydraulic shock absorber 101 due to a resultant flow resistance of the oil. In addition, some of the oil in the lower chamber S2 flows from the side hole 137 of the piston rod 103 to the oil path 136, and then into the upper chamber S1 through the seating member 141, resulting in generation of damping force in the hydraulic shock absorber 101 due to the resultant flow resistance of the oil. This damping force is adjusted by moving the push rod 131 up and down by means of the electric motor 134 and the feed screw mechanism 135 so as to change an opening area of the seating member 141. Specifically, pushing the push rod 133 upward leads to reduction in the opening area of the seating member 141, increasing a flow resistance of the oil passing through the seating member 141. As a result, the generated damping force is adjusted high. On the other hand, moving the push rod 133 downward leads to expansion of the opening area of the seating member 141, reducing the flow resistance of the oil passing through the seating member 141. As a result, the generated damping force is adjusted low.
One of the problems with this hydraulic shock absorber 101 having such damping force adjustment unit is that downward reaction force (a direction of an arrow in the view), generated by a pressure of the oil, acts on the push rod 133 and then on the electric motor 134 via the feed screw mechanism 135, and when the reaction force acting on the push rod 133 becomes extremely strong due to an increased speed of the piston, such reaction force exceeds thrust of the electric motor 134, causing desynchronization in the electric motor 134 and interfering normal adjustment of the damping force.
In this regard, Patent Literature 2 (JP-A-2013-007408) proposes a hydraulic shock absorber in which is defined a desynchronization section where desynchronization in the stepping motor is likely to occur if the push rod (a valve body) is moved farther toward the seating member (a valve seat), wherein, in the desynchronization section, the stepping motor is energized so as to move the push rod more than twice the distance to the seating position.